Flexible resistance element with flexible and stretchable terminal electrodes



p 1967 D. M. BARTOS ETAL 3,34 ,385

FLEXIBLE RESISTANCE ELEMENT WITH FLEXIBLE AND STRETCHABLE TERMINALELECTRODES Filed Jan. 4, 1965 'IIIIIII'II/IIII' Q in INVENTORS. Dona/0M4fiar/os Y Raymondd Price 3 H TTORNEV United States Patent 3,344,385FLEXIBLE RESISTANCE ELEMENT WITH FLEX- IBLE AND STRETCHABLE TERMINALELEC- TRODES Donald M. Bartos, Midland, and Raymond J. Price, Bay City,Mich, assignors to Dow Corning Corporation, Midland, Mich, a corporationof Michigan Filed Jan. 4, 1965, Ser. No. 423,228 2 Claims. (Cl. 338212)ABSTRACT OF THE DISCLOSURE Flexible and stretchable elas-tomericresistance element, having flexible and stretchable braided, open weavemetallic terminal electrodes, which have the capacity to maintainintimate contact with the elastomeric, resist ance element duringflexing, bending, and stretching of the element while it is in use. Suchintimate contact between an elastomeric resistance element and itsterminal electrodes is necessary if current is to flow uniformly throughthe resistance element.

The present invention relates to electrically conductive flexible orstretchable materials, and more particularly, to the provision ofterminal connections for such materials.

Various types of electrically conductive plastic and elastomericmaterials have been proposed. Generally, such materials are formed ofelectrically conductive particles suspended in a-natural or syntheticrubber, or plastic, flexible or stretchable carrier. By proper selectionof materials it is now possible to provide flexible and/or stretchablematerials having any balance of electrical and physical properties. Suchdevices have potential application in fields such as resistanceelements, heating elements, tapes, pads, blankets, clothing and manyothers.

A major problem heretofore, with such materials, has been the diflicultyin attaching suitable electrodes to the material for connection to anelectrical power source. Electrodes must have intimate contact with aresistance element throughout its entire length. The ability to maintainintimate elecrode contact during flexing, bending, and stretching of theelement in use is considered by many design engineers to be the mostimportant criterion in selecting such devices for a particular use. Thishas been particularly true in heating applications wherein a relativelylarge area is to be uniformly heated. If electrodes are attached only atpoints on the electrically conductive resistance element, electricalcurrent is distributed nonuniformly through the area resulting in hightemperatures (or hot spots) in areas of high current concentration andlow temperatures in areas remote from the electrodes. It is thereforedesirable to provide an electrode arrangement wherein current is causedto flow uniformly through the resistance element. It is toward thisobject that the present invention is directed.

Another object is the provision of an electrode, which is capable ofstretching and yielding with the flexible resistive element forrelatively wide stretchable electrical elements. A further object is toprovide a method of making a wide flexible conductor having yieldableelectrodes made integrally therewith.

In accordance with these and other objects, there is provided by thepresent invention a yieldable, stretchable, low resistance electrode,which is bonded to a flexible and/or elastomeric electrical resistanceelement. Resistance elements may be either supported or unsupported. Theelectrode may, for example, be a stretchable metallic conductor which isprocessed into the resistance element before the element has beenvulcanized or cured. If desired, flexible electrical insulation can bebonded over both the resistance element and the electrodes.

Other objects and many of the attendant advantages of the presentinvention will become apparent to those skilled in the art by aconsideration of the following detailed description When read inconjunction with the acccompanying drawings wherein:

FIG. 1 is a view in perspective of a heating pad made in accordance withthe present invention;

FIG. 2 is a cross-sectional view of the heating pad of FIG. 1 taken onthe line 2--2 of F IG. 1;

FIG. 3 is a view in perspective of the heating element ofFIG. 1; and

FIG. 4 is a diagrammatic view illustrating a method of making insulatedelements in accordance with the present invention.

Referring now to the drawings wherein like reference numerals designatelike or corresponding parts throughout the several views, there is shownin FIG. 1 a heating pad shown generally as 10. The heating pad iscovered by a layer of insulating material 11, and has a pair ofelectrodes 12 and 13 embedded therein, and projecting from the pad forconnection to an electrical power source. It will be realized that inpractice the leads emerging from suitable locations on the pad will alsobe insulated to prevent short circuiting and danger to the user.

As may be seen from FIGS. 2 and 3, the electrodes are embedded in alayer of flexible conducting material 14 which may be of any known typewhich is in turn covered by a layer 11 of insulation. Numerous examplesof suitable conductive materials are described in copending US. patentapplication, Ser. No. 408,263, filed Nov. 2, 1964. The material maymerely be flexible, or it may be elastomeric in nature. It is preferablethat the material be capable of flowing or being made to flow at sometime in the manufacturing process and be capable of bonding or adheringto the electrodes.

It is desirable that the electrodes be capable of flexing with theconducting material without loss of bond or intimate contact. Forexample, if the conducting material is elastomeric in nature, theelectrode must be capable of stretching or yielding with that materialduring flexing or stretching. A suitable type electrode is an open weaveor braided metallic conductor such as the type of braid commonly used asa shield in audio equipment such as microphone cables. In a specificembodiment, an electrode consisting of 50 strands of tinned copper wirebraided in tubular form was utilized. Before embedding it, the electrodewas expanded to render it more stretchable and flattened to conform tothe flat pad. The open weave braided electrode is particularly desirabledue to the large surface area of the conductors, high currentcapability, and assurance of excellent bonding since the material of theresistance element can be flowed through the open weave. While it isalso possible to use conductive elastomeric materials, the resistance ofmost such materials is too high due to a voltage drop along theelectrode to obtain uniform current distribution.

For uniform heating in a resistance element having substantially uniformresistance per unit of volume, it is desirable to provide current pathsof equal length throughout the material. The heat generated by theconducting substantially equal current distribution and substantiallyequal resistance will thus result in uniform temperature.

Uniform current distribution is accomplished in accordance with thepresent invention by extending the electrodes 12 and 13 throughsubstantially the entire length of the heating element in paralleldirections as shown in FIG. 3. Since the distance between electrodes issubstantially uniform, the heat produced will also be substantiallyuniform.

Patented Sept. 26, 1967- As shown in FIG. 4, insulated heating elementsmade in accordance with the present invention may easily be massproduced. Assuming that the conductive material used as a resistanceelement is an elastomeric such as a conductive dispersion in a siliconerubber carrier, a web 14 of uncured or tacky conducting material, whichmay be supported, if desired, is fed between a first pair of pressurerollers 16 and 17. Electrode material 13, held, for example, on a reel18 is also fed into the space between pressure rollers 16 and 17 in thepositions at which it is to be embedded into the conducting material 14.The pressure rollers force the electrode material into the uncuredconductor which, because of its uncured state, is caused to flow intotight adherence with the electrodes. A pair of webs 11 and 11a ofinsulating material, also preferably, but not necessarily, in uncuredform are fed with the conductive web into the space between a secondpair of pressure rollers 19 and 20. The insulating jacket could besilicone rubber, for example. The insulation will adhere well to eithercured or uncured conductive material. The composite web is then fedthrough a curing oven 21, where it is cured as necessary, depending onthe type of material used, and is then wound on a takeup reel 22, whereit may be stored until ready for use. It is to be understood that, whilenot as desirable in many applications, the materials may be cured beforeassembly and conductive adhesives may be flowed through the electrodesto secure them to the webs.

Obviously, the insulated heating elements need not be produced in rollsas shown in FIG. 4. However, the same basic steps are involved in makingsingle small elements. The braid electrodes are forced into an uncuredsheet of conductive material. The conductive material may, if desired,be provided with fabric reinforcement. Insulation, if desired, is placedover the conducting material and cured as necessary.

Many other modifications and variations of the present invention willbecome obvious to those skilled in the art. Therefore, it is to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

That which is claimed is: 1. A flexible resistance element comprising anelastomeric body of flexible electrically conductive material, and asystem of flexible and stretchable low resistance electrodes embeddedtherein, said embedded electrodes each having a length in said materialcorresponding substantially to the length of said material in thedirection in which said electrodes are embedded, and said embeddedelectrodes being each formed as a braided open weave metallic conductor.2. A resistance element as defined in claim 1, but including further alayer of insulating material surrounding said elastomeric body offlexible electrically conductive material and said low resistanceelectrodes.

References Cited UNITED STATES PATENTS 2,406,367 8/1946 Griffith et al.244l34 2,473,183 6/1949 Watson 219549 X 2,559,077 7/1951 Johnson et al.219-549X 2,885,461 5/1959 Cafiero 3159-222X 2,952,001 9/1960 Morey3382l0 3,022,412 2/ 1962 Waters 219-549 3,060,303 10/1962 Skoglund etal. 219-549 3,281,579 10/1966 Glicksman 219-535 FOREIGN PATENTS 975,26411/ 1964 Great Britain.

RICHARD M. WOOD, Primary Examiner.

V. Y. MAYEWSKY, Assistant Examiner.

1. A FLEXIBLE RESISTANCE ELEMENT COMPRISING AN ELASTOMERIC BODY OFFLEXIBLE ELECTRICALLY CONDUCTIVE MATERIAL, AND A SYSTEM OF FLEXIBLE ANDSTRETCHABLE LOW RESISTANCE ELECTRODES EMBEDDED THEREIN, SAID EMBEDDEDELECTRODES EACH HAVING A LENGTH IN SAID MATERIAL CORRESPONDINGSUBSTANTIALLY THE LENGTH OF SAID MATERIAL